The multilayer ceramic (MLC) technology for fabricating dielectric substrate carriers for semiconductor integrated circuit package assemblies is well-known. The MLC substrate is fabricated by laminating as many as 32 layers of metallized ceramic sheet (formed from alumina mixed with binder material) and sintering the laminate at a high temperature. Each ceramic sheet has a metallic (typically, molybdenum) pattern printed on it, and holes are punched into the sheet to permit metal interconnections (vias) between printed-circuit layers. The sintered substrate is employed for mounting active devices, such as semiconductor chips. In large scale integrated circuit packages, it is known to provide a MLC substrate with engineering change (EC) pads in addition to the controlled collapse chip connection (C-4) pads to be solder bonded to the semiconductor chips. This concept is described in more detail in U.S. Pat. Nos. 3,726,002, 3,968,193 and 4,221,047 assigned to IBM Corporation, the present assignees and in the IBM Technical Disclosure Bulletin, Vol. 15, No. 8, Jan. 1973, p. 2575. In use, discrete wires are ultrasonically bonded to the EC pads which provide additional or alternate wiring capable of connecting the various C-4 pads of the chips mounted on the substrate.
The chip mounting is generally accomplished using a "flip-chip" orientation whereby the chips are mounted to the C-4 pads on the substrate surface using a solder (typically, lead-tin) reflow process. In order to achieve a good bond for the lead-tin solder, the chip mounting C-4 pad is frequently prepared with a very thin coat of gold on a thin coat of nickel deposited over the molybdenum via metallurgy. U.S. Pat. No. 4,493,856 to Kumar et al and assigned to the present assignee (which is hereby incorporated by reference herein), discloses a dual-material metallization process applied to both the C-4 and EC pads. As discussed therein, nickel has excellent adhesion to molybdenum and the subsequent thin flash layer of gold prevents oxidation of the nickel. In addition, the very thin coating of gold on the C-4 pads allows for a good solder bond for chip mounting. A heavy concentration of gold on the C-4 pads, however, leads to solder wettability problems which result in reliability fails upon thermal cycling. The nickel and goldtreated EC pads, on the other hand, require additional heavy plating with gold to allow for frequent and repeated changes in the wire bonding to the pads thereby accommodating testing, engineering changes and defect compensation.
An electroplating method method for forming heavy gold plating on MLC substrates is described in IBM Technical Disclosure Bulletin, Vol. 20, No. 5, October 1977, p. 1740. Applying heavy gold on these pads by a plating process has a history of problems. At times, the heavy gold blisters, at other times, adhesion of gold is poor. U.S. Pat. No. 4,526,859 to Christensen et al and assigned to the present assignee (which is hereby incorporated by reference herein) discloses use of photoresists as masking layers in obtaining a heavy coating of a metal (gold) on either the EC or C-4 pads. The use of resists as mask material is well known, see for example, U.S. Pat. No. 3,957,552 to Ahn et al and Japanese application No. 50-124930, Apr. 19, 1977. As in Christensen, et al, these references disclose the application of a resist, selective exposure of the resist using an appropriate mask and development of the exposed resist forming a pattern and revealing the underlying surface intended to be metallized. Metallization of the entire surface follows whereby the metal layer is deposited on the unexposed resist and on the patterned underlying surface. Removal by float-away or etching techniques of the remaining resist with the overlying metal results in a metallization pattern on the surface.
Similarly, metal masks can be used by placing them in registration with the substrate and, essentially, screening through the mask. However, it is difficult to achieve registration of a pre-formed metal mask with an MLC substrate which has undergone uneven shrinkage during sintering.
Another known method of selectively depositing a metal coating on a pre-existing conductive pads (EC or C-4) requiring no masking step is the maskless cladding process described in U.S. Pat. No. 4,442,137 to Kumar and assigned to the present assignee. In this method, a blanket metal coating is deposited by sputtering, vapor deposition of other known process and subsequently heated to a temperature sufficient to cause the overlying, e.g., gold, coat to diffuse with the underlying metallurgy. At the same time as the metal-to-metal diffusion is occurring, stresses occur which are sufficient to cause the metal deposited on the substrate to flake or spall and consequently be readily removable in a follow-up mechanical cleaning step, such as ultrasonic removal of the residue. However, the deposition and diffusion is nonselective and therefore causes the heavy overlying metal to diffuse and adhere to all of the metal interconnection pads. EC and C-4 alike. As noted above, it is desirable to have a thick gold coating on the EC pads, but not on the C-4 pads.
By far the most promising of all known prior art process for depositing metal onto a selected portion of a metallurgical pattern is disclosed in U.S. Pat. No. 4,582,722 to Herron et al and assigned to the present assignee (which is hereby incorporated by reference herein). In this process (known as the cladding process) a barrier layer is utilized to isolate the areas of underlying metallurgy on which additional metal coating is not desired. The barrier layer disclosed therein is comprised of a ceramic particulate paste having a polymer binder and a low vapor pressure solvent. The barrier layer is allowed to dry to expel the solvent and is baked in a reducing atmosphere to expel the polymer binder. The remaining inorganic layer, having no organic or carbonaceous residues, has sufficient strength to withstand the subsequent maskless cladding processing steps of metal deposition, diffusion and patterning to remove the metal from the non-metallic substrate areas. During the "patterning", such as by ultrasonic means, the barrier layer is also removed leaving a selectively metallized surface.
Although the method of Herron et al, supra has proven to be a highly reliable cladding process, it nevertheless suffers from the disadvantage of requiring a polymeric material to "glue" or hold together the particles constituting the paste. Removal of the paste after screening and drying necessitates subjecting to a high temperature in the range of about 350.degree.-450.degree. C. Such high temperature treatment may result in a high surface concentration of nickel, depending on the thickness of the overlying layer of gold, in the nickel-treated EC pads thereby deleteriously affecting the reliability of the EC pad. From a health and environmental standpoint, due to the generally volatile nature of the polymeric material used in the paste, the handling of such a paste, even in a highly ventilated manufacturing set up, is unpleasant. Also, removal of any nondecomposed polymer (for example, amyl acetate) used in the paste may require a vigorous solvent (such as trichloroethane or xylene) further aggravating the situation. Particularly from the standpoint of cleanability of nondecomposed polymeric material in the paste, since ultrasonic methods are generally employed for cleaning in a large volume manufacturing environment, in order to safeguard against health and explosion hazards associated with the solvents used for cleaning, construction of a complicated ultrasonic cleaning tank system will be necessitated. Such a system adds to the manufacturing cost of the product.
Another basic shortcoming of the prior art polymer-based paste for cladding process is that, due to its fundamental requirement that the substrate be subjected to a high temperature to decompose the binder material in the paste, it cannot be used in conjunction with a substrate which itself is composed or has a coating thereon of a thermally degradable material. One such situation is the fabrication of advanced chip packaging known as thin film redistribution (TFR), wherein one or more layers of metallization and insulating material are deposited onto a MLC substrate, each layer being patterned with a photolithographic process. The metallization layers themselves can consist of several different metals sputtered in succession. In the TFR, it is desirable that the EC pad metallization have an additional sputtered layer of metal that the C-4's do not have. One way of putting down an extra layer of metal exclusively on the EC pads is by using a time-consuming and expensive new photolithography cycle of deposition, exposure, development, sputtering, and removal. A more expedient way of selectively metallizing the EC's in a TFR context is by selectively applying a paste suitable as a masking material on the EC pads. In this context, the prior art polymer-based paste would be unsuitable as a masking material since the high temperature thermal step to decompose the polymeric content of the paste would also tend to decompose the photosensitive polymer layers formed on the substrate thereby destroying the TFR structure. Even if the photosensitive polymer layer is not destroyed at this high temperature, interdiffusion of the layers of the metallurgy which ruins the functionality of one or more metallization layers cannot be avoided.
Accordingly, it is an object of the invention to provide a screening paste which does not necessitate a high temperature heating prior to its removal.
It is another object of the invention to provide a screening paste which is free of polymeric material.
It is yet another object of the invention to provide a barrier layer of material for selective masking of a metallization which is conducive to easy removability and cleanability.
It is still another object of the invention to provide a barrier layer of material for covering a selected portion of a metallization on a supporting substrate which is either itself composed of an organic polymeric material or contains one or more layers of such material.